Advances in microelectronics necessitate components with ever smaller critical dimensions. Manufacturing such components requires the use of shorter wavelengths of light in the lithography processes employed in component fabrication. This, in turn, has lead to a need to measure the optical characteristics of samples such as, among other, photolithographic masks and fabricated components over a broad range or band of wavelengths including the UV. Typically, in these measurements the cross-sectional diameter of a beam of light focused on the sample is large enough to spatially average the optical characteristic being measured yet small enough to resolve spatial variations across the sample. As the critical dimensions have decreased so too has the required diameter of the beam of light on the sample. It is now desirable to have a diameter of less than 100 micron.
As with many engineering problems, the design of an optical system to measure the optical characteristics of such a sample represents a tradeoff. For example, when illuminating the beam of light with the broadband of wavelengths from a light source onto a surface of the sample, it is desirable to have a small spot size but not a diffraction limited spot. In addition, this should be accomplished in an optically efficient manner. There is, therefore, a tradeoff in this regard between a need for optical components with a low f-number (for higher optical efficiency) and a need for optical components with a high f-number (for a small spot size over a practical depth of field with minimal aberration and angles of incidence) and thus a small cone of rays corresponding to the beam of light that is used in the optical system, i.e., the useful light. Similar design tradeoffs occur in the collection and illumination on a detector of the beam of light reflected from the sample and the beam of light transmitted through the sample.
The need to operate over the broadband of wavelengths is a further design constraint for many optical components because they are subject to a variety of effects such as chromatic aberration and absorption. For refractive optical components these effects become pronounced as the wavelengths approach the UV. There exist optical systems based on refractive optical components in the prior art that operate over a broadband of wavelengths with a small diameter of the beam of light on the sample. In these systems, attempts are made to compensate for chromatic aberration and absorption effects. However, this adds expense and complexity to these optical systems.
Reflective optical components are a suitable solution to this technical challenge. A wide variety of components are available including mirrors with non-spherical shape, such as an off-axis paraboloid shape, henceforth called an off-axis parabolic mirror. However, non-spherical shaped mirrors can add expense to the optical system, especially when such mirrors are manufactured by diamond turning. Optical systems including torroidal, spherical and elliptical mirrors are disclosed in the prior art. For examples, see U.S. Pat. Nos. 5,910,842, 6,583,877 and 6,128,085.
In addition, many prior art broadband optical systems combine refractive and reflective optical components. However, such catadioptric systems do not avoid the complexity and expense needed to overcome the chromatic aberration and absorption issues associated with refractive optical components.
Furthermore, when different samples are characterized, the beam of light in the optical system will need to be focused on the sample to correct for effects such as varying surface topography. Such an adjustment is problematic if the adjustment of the position of certain optical components in the optical system necessitates the adjustment of the position of many other optical components, since this can easily lead to misalignment. A preferred solution would allow the beam of light to be focused on the sample by adjusting a minimum number of components in the optical system or a simple assembly of components. Furthermore, such a preferred solution would be a sufficiently compact and simple optical system that a single light source could be used to optically characterize the reflection and transmission properties of the sample.
There is a continued need, therefore, for a compact optical system for optical characterization of a sample, which operates over a broadband of wavelengths with a small diameter of the beam of light on the sample and which employs reflective optics with a minimum number of optical components such that advantageous components such as off-axis parabolic mirrors can be used. There is also a need for such an optical system that can be focused by adjusting the position of the minimum number of optical components or a simple assembly of components.